Preheat the oven at 180 C ten minutes prior to baking.
In a large blender add overripe bananas. To it add Greek yogurt, vanilla extract, eggs, oil and sugar. Blend on high speed for a minute or till everything comes together. Keep aside.
In a separate large bowl, sieve wholewheat flour 2-3 times. Also, sieve baking powder, baking soda and salt. Mix it all well.
Now add chopped walnuts to the flour. Mix them well so that walnuts are coated with flour.
Now make a well in the center of flour mixture and add the wet batter in 3 to 4 batches. Fold the dry ingredients to wet ingredients very well. Gently cut and fold the batter and do not whisk in circular motion.
Next fold in dark chocolate chunks or dark chocolate chips to the batter. I also like to add some on the top.
Line a standard 8 by 9 inch circular pan with a parchment paper or alternatively grease baking tin with oil and dust it with some flour.
Bake for 35 minutes or till toothpick inserted at the center comes out clean. Allow it to cool for 15 minutes in the pan.
To remove the cake from the pan, simply run a blunt knife around the edges and invert on the wire rack. Tap the pan 2 or 3 times and the cake will slide out.
Slice and serve when the cake is warm or has completely cooled.